Used to be powered internally (4)
I believe the answer is:
were
'used to be' is the definition.
(eg 'you were...' means 'you used to be...')
'powered internally' is the wordplay.
'internally' says the answer is hidden in the clue.
'WERE' is hidden within 'powered'.
(Other definitions for were that I've seen before include "Existed in the past" , "Lycanthrope" , "SECOND PART OF 9" , "(We/you) existed" , "'Are, in the past (4)'" .)